..or deinhibitor.
Reuter's medical dictionary: Enthemmung = disinhibition

I agree that logically, the overall effect is probably activation. However, depending on context, it may be important from a pharmacological perspective to qualify that the activation is a result of the removal of some inhibitory factor (e.g. enzyme blockade by another drug is removed, thus causing activation) rather than "simply" activated as a direct effect of the action being described. Otherwise the author may not have used such a distinct term.

In biochemistry neither term is common, though both are used in this context in journal articles that turn up in Google (ignoring the psychology references).